The api module previously had defaultPartition and defaultNamespace vars
for when we need default/empty split usage between ent/oss respectively.
This commit moves those two variables out of test code so that they can
be used for the service exports config entry's `GetNamespace()` method.
Previously `GetNamespace()` would return "default" in both OSS and enterprise,
which can trip up automation that passes the result of this method as the
namespace to write a config entry.
The split vars are kept private to prevent external usage, and prefixed with
`split` for more clarity about their behavior.
